1. Field of the Invention
This invention relates to a lens barrel of an inner focusing type for use in a video camera. More particularly, this invention relates to a lens barrel having reference shafts for movably supporting lenses.
2. Description of the Background Art
Conventionally, there have been proposed various lens barrels for use in optical systems such as video cameras. Among such proposed lens barrels, a lens barrel of an inner focusing type is known in which a plurality of lens groups including a master lens group are adjustably moved along an optical axis thereof. FIG. 1 depicts such a conventional inner focusing lens barrel. Referring to FIG. 1, the lens barrel 100 has a cylindrical front housing member 101, an approximately annular iris block 102 and a cylindrical rear housing member 103, each of which is made of synthetic resin. A rear end portion of the front housing member 101 is connected by a screw 116 to a front end portion of the iris block 102. A rear end portion of the iris block 102 is connected by a screw 117 to a front end portion of the rear housing member 103. The front housing member 101 has a jaw 101a which extends radially inwardly from a periphery thereof so as to retain a lens group 104 used as a front lens. Furthermore, the iris block 102 retains a stationary lens group 105 in an inner periphery thereof.
A pair of guide shafts 106, 106 is supported between the front housing member 101 and the iris block 102 and extend parallel to an optical axis C. Similarly, another pair of guide shafts 107, 107 is supported between the iris block 102 and the rear housing member 103, extending parallel to the optical axis C.
The guide shafts 106, 106 pass through a first lens holder ring 109 such that the lens holder ring 109 is slidably supported thereon for axial slidable movement. The lens holder ring 109 retains a variator lens group 108 relative to the optical axis C which fulfills a variable zoom function. Further, the guide shafts 107, 107 pass through a second lens holder ring 111 such that the lens holder ring 111 is slidably supported thereon for axial slidable movement. The lens holder rings 111 retain a master lens group 110 which fulfills focusing, compensating and image-forming functions.
The first lens holder ring 109 is provided, on an outer periphery thereof, with a groove 109a acting as a cam follower. A projection 112a is engaged into the groove 109a and extends spirally along an inner surface of a cam drum 112 which is slidably rotated within the front housing member 101 by a stepping motor (not shown). The rotating movement of the cam drum 112 causes the lens holder ring 109 having the groove 109a to slide along the spiral projection 112a of the cam drum 112. Accordingly, the lens holder ring 109 reciprocally moves along the guide shafts 106, 106 so that the variator lens group 108 retained by the lens holder ring 109 moves reciprocally axially to and fro along the optical axis C.
The second lens holder ring 111 is supported through a support member 113 by a rotating shaft of a stepping motor 114 which projects forwardly from a housing body of the motor 114. The support member 113 is threadedly engaged with a lead screw 115 which is formed in a forward portion of the rotating shaft of the motor 114. When the motor 114 is actuated, the screw 115 rotates so that the support member 113 moves to and fro along the screw 115. Therefore, the second lens holder ring 111 held by the support member 113 reciprocally moves forward or backward along the guide shafts 107, 107 extending parallel to the optical axis C. To this end, the master lens group 110 retained by the lens holder ring 111 moves to and fro in a direction axially parallel to the optical axis C.
The iris block 102 further includes a plurality of diaphragm blades 119 which are operated via a rotating shaft 118a of a motor 118 so as to dilate or contract for adjusting an amount of light to the stationary lens group 105. In FIG. 1, numerals 120, 121 and 122 represent a low-pass filter, a charge coupled device (CCD) and a CCD substrate, respectively.
As described above, conventional lens barrels are divided into three parts, namely, the front housing member 101, the iris block 102 and the rear housing member 103. The respective parts formed of molded resin material are provided with socket and spigot joint sections, projections or pins which, upon assembly, serve for positioning of lenses in the lens barrel relative to the optical axis C. In order to limit positioning error to an allowable 15-20 .mu.m, the parts must be manufactured to tolerances within a few microns. However, it is difficult to realize such small tolerances according to present molding techniques.
Further, conventionally, the guide shafts 106, 106 and 107, 107 for supporting the variator lens group and the master lens group 110 are separately secured within the front and rear housing members 101 and 103 of the lens barrel 100. Therefore, the lens group 104, 108, 105 and 110 retained in the lens barrel 100 may be decentered or inclined relative to the optical axis C during assembly of the lens barrel. Also, the total number of the parts of the lens barrel is large so that the lens barrel is complicated, with an enlarged structure.
Thus, such conventional lens barrels have been difficult to manufacture with sufficient precision while yet using a limited number of parts.